* {
    margin: 0;
    padding: 0;
    list-style: none;
}
body {
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
}
.box {
    width: 400px;
    height: 300px;
    overflow: hidden;
    position: relative;
}
.imgsum {
    width: 2000px;
    height: 300px;
    // transition: 5s;
    animation: img 5s ease-out infinite normal;
    ul li img {
        float: left;
        width: 400px;
        height: 300px;
    }
}

@keyframes img {
    0% {
        margin-left: 0px;
    }
    25% {
        margin-left: -400px;
    }
    50% {
        margin-left: -800px;
    }
    75% {
        margin-left: -1200px;
    }
    100% {
        margin-left: -1600px;
    }
}
